Allow users to opt out of including the layer tools in a layered jar

For Maven, the layer configuration is now an additional configuration
option instead of a layout type.

Closes gh-19866

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootJarIntegrationTests.gradle

@@ -11,6 +11,8 @@ bootJar {
 		}
 	}
 	if (project.hasProperty('layered') && project.getProperty('layered')) {
-		layered()
+		layered {
+			includeLayerTools = project.hasProperty('excludeTools') && project.getProperty('excludeTools') ? false : true
+		}
 	}
 }